Factor IX is a vitamin K-dependent blood clotting protein that is deficient in the inherited disorder hemophilia B. Treatment of patients with hemophilia B has been performed with plasma, and factor IX concentrates of intermediate and high purity. Treatment of patients with hemophilia B has been complicated by infections including the human immunodeficiency virus and hepatitis A, B and C, resulting in significant treatment related morbidity and mortality. In addition patients receiving intermediate purity concentrates have suffered disseminated intravascular coagulation and both venous and arterial thrombosis. While high purity products have not yet been associated with thrombosis, the risk of infection remains a concern. Recombinant factor IX was first developed on a laboratory scale in the late 1980s. The goal of this project is to administer rhFIX to patients with hemophilia B. After an initial pharmacokinetic evaluation, treatment based on 2 dosing strategies will be tested: as needed for the treatment of bleeding or as prophylaxis.